This invention relates to a system for the regeneration, by oxidation, of the particulates deposited in a particulate filter trap of an internal combustion engine provided for purifying the engine exhaust gas.
Particulate filter traps are utilized for purifying engine exhaust gas in order to reduce particulate emissions from internal combustion engines, such as diesel engines having direct or indirect injection. Such filter traps, of honeycomb-like, monolithic and ceramic material, have a high filtering effect for the soot in the exhaust gas from diesel engines. The honeycomb filter traps have a plurality of inlet channels along which the exhaust gas to be purified flows, the greatest amount of soot and other particulates being deposited on the internal ceramic walls of the channels, and the purified gas thereafter flowing outwardly through outlet channels. The channels are parallel to one another with the inlet channels being closed at the outlet face of the filter, and the outlet channels being closed at the inlet face of the filter. As the soot is deposited on the surfaces of the inlet channels, the flow resistance through the filter increases which leads to an increase in the exhaust gas counterpressure. Such an increase in exhaust gas counterpressure, however, effects an increase in fuel consumption and can, in the event of a blockage in the filter, cause engine stall. It therefore becomes necessary to continuously or intermittently burn away the soot and other particulates collected in the filter. Temperatures of 550.degree. to 650.degree. C. are required for thermal soot oxidation. By heating up the entire exhaust gas flow these temperatures can be thereby obtained. However, with a low amount of heat removal, it is possible to employ an initial ignition to burn the soot at the beginning of the filter inlet channels such that during the course of the soot being exothermally burned off, a further and self-sustained combustion of the soot layer can be effected.
Besides obtaining the increased temperature acquired for thermal soot oxidation, a self-sustaining regeneration of the particulate filter trap for completing the soot burn-off in the inlet channels depends especially on the amount of soot deposited in the inlet channels. A sufficiently thick layer of soot must be present in the inlet channels so that, because of heat released by engine combustion (minus the heat that has been removed by the exhaust gas and the filter material), a positive heat balance for the propagation of the soot burning may be effected. However, the thickness of the deposited soot layer must not be too great since the heat development otherwise increases to such an extent during soot combustion which results in destruction of the filter due to "heat bonding" or "melting on" of the filter material and/or due to thermal cracking resulting from impermissibly high temperature gradients over the filter material.
In the regeneration process according to U.S. Pat. No. 4,436,535, the pressure in front of the filter is measured. Pressure measured in such manner, however, is dependent on the velocity of flow of the exhaust gas in the filter channels. The flow velocity is dependent on the exhaust gas temperature and the exhaust gas volume flow, and hence upon load (torque) and the rpms of the engine. Furthermore, the counterpressure is influenced by the uneven deposit of the soot as well as the soot composition and thereby the density of the soot layer.
Sensors may also be employed for measuring the particulate concentration in the exhaust gas stream which is to be purified so as to use it as a control size. The disadvantage in such an approach, however, is that the particulate accumulatation precipitated on the sensors is dependent on particle size and the composition of the particulates. The control of the regeneration operation should rather take place independently of the flow velocity, the particulate size and the composition of the particulates.
U.S. Pat. No. 4,283,207 discloses an approach in which a periodic ignition of the particulates is to take place by applying a relatively strong electric current and voltage through the soot layer for combustion thereof. The soot ignition and burn-off step is therefore dependent not only on the amount of electrical energy conducted into the soot layer, but rather most importantly on the removal of heat by the exhaust gas which is to be purified by the filter trap material being employed. Here, heat removal by means of the exhaust gas is a function of load (torque) and the engine rpms. This leads to the disadvantage that the burning of soot is introduced with greatly varying amounts of soot present as a consequence of the dependence on the operation of the engine which, in this case, is undesireable. Another disadvantage is that due to the permanent contact made with the voltage, which is not restricted to given periods, an ineffective and high consumption of electrical energy takes place.
U.S. Pat. No. 4,549,398 discloses an exhaust gas cleaning device for diesel engines as including a heater, a detection signal line at the inflow side of the filter for detecting exhaust gas pressure, a detection signal line for detecting the temperature inside the filter, and a detection signal line at the outflow side of the filter for the temperature of the exhaust gas. However, the heat capacity for the soot layer cannot be detected with such arrangement, since a second in tandem or sequentially arranged heat sensing element is absent. Instead, this prior art device utilizes the temperature measuring positions to determine favorable marginal values for starting the filter regeneration. For this purpose the amount of soot in the filter is also detected. However, detection is carried out, not by measuring the heat capacity as in the invention, but rather by measuring the flow resistance of the soot layer by means of a pressure measurement at the inflow and outflow sides of the filter.